Monto, A. S. and S. K. Lim (School of Public Health, Univ. of Michigan, Ann Arbor, Mich. 48104). The Tecumseh study of respiratory illness. III. Incidence and periodicity of respiratory syncytial virus and Mycop/asmo pneumoniae infections. Amer J Epidem 94: 290–301, 1971.—Families living in Tecumseh, Michigan were successively placed on surveillance for acute illnesses for a period of one year. Blood specimens were obtained from family members at 0, 6 and 12 months of surveillance. Specimens collected in 1966 and 1967 were tested for rises in antibody titer to respiratory syncytial (RS) virus and Mycoplasma pneumoniae. The temporal patterns of rises in titer for RS virus indicated that the virus had been transmitted in both years for a relatively limited period. Individuals with rises in RS antibody titer had significantly more respiratory illnesses than matched controls during the periods of prevalence of the virus. With M. pneumoniae, rises in antibody titer demonstrated some transmission in the first portion of 1966 and little thereafter. Three times as many fourfold falls as compared with rises in titer were found, indicating that much transmission had occurred before the start of the study. During periods of RS virus prevalence, admissions at Tecumseh's only hospital for pneumonia, bronchitis and bronchiolitis among small children failed to increase. This suggests that RS virus may produce a relatively milder illness in an open, small town population than in the inner cities or closed populations of children. For both RS virus and M. pneumoniae the highest rates of serologically detected infection occurred among the 5- to 9-year-olds, a group ordinarily heavily involved with respiratory infections.